1. Field of the Invention
The present invention relates to a multilayer wiring board and a method of fabricating the same and, in particular, to a multilayer wiring board comprising a plurality of conductor patterns stacked with an insulating layer of a thermosetting resin interposed between adjacent conductor patterns, and a method of fabricating the multilayer wiring board.
2. Description of the Related Art
Various multilayer wiring boards each comprising a plurality of conductor patterns stacked with an insulating layer of a thermosetting resin interposed between adjacent conductor patterns are proposed in Japanese Unexamined Patent Publication No. 10-190232, etc.
These multilayer wiring boards, as shown in FIG. 7A, are each formed by integrally stacking a plurality of resin sheets 100 of a thermosetting resin which exhibits the bonding properties by heat treatment at a predetermined temperature.
Each resin sheet 100 includes a thermosetting resin layer 102 capable of exhibiting the bonding properties by heat treatment at a predetermined temperature, a conductor pattern 104 formed on one surface of the thermosetting resin layer 102, and at least a via 108 formed through the thermosetting resin layer 102. The via 108 has an end thereof connected to the conductor pattern 104 and the other end thereof exposed at a second surface of the thermosetting resin layer 102. The via 108 is formed of a recess 106 open to the second surface of the thermosetting resin layer 102 and having the bottom surface thereof exposed to the reverse surface of the conductor pattern 104, which recess 106 is filled, by electroplating, with a solder adapted to melt at a temperature lower than the temperature at which the thermosetting resin layer 102 exhibits the bonding properties.
A plurality of the resin sheets 100 shown in FIG. 7A are stacked and heat treated at a temperature enabling the thermosetting resin layers 102 to exhibit the bonding properties, whereby the multilayer wiring board shown in FIG. 7B can be produced. In the multilayer wiring board shown in FIG. 7A, the conductor patterns 104 stacked in multiple layers are electrically connected to each other by the vias 108.
The multilayer wiring board shown in FIG. 7B is configured to improve the dimensional and positional accuracy of the vias, etc. and can be easily reduced in thickness with the conductor patterns formed at a high density.
The resin sheets 100 constituting the multilayer wiring board shown in FIG. 7B, however, are each substantially formed of a thermosetting resin and therefore thermally expand/contract to a greater degree than package members mounted thereon such as a semiconductor element. It has been found, therefore, that the improvement in the reliability of connection between the multilayer wiring board and the package members such as a semiconductor element mounted thereon is limited.
In order to suppress the expansion/contraction of the multilayer wiring board, an attempt has been made to fabricate a multilayer wiring board using a resin sheet having arranged therein a reinforcing member composed of unwoven fabric such as glass cloth or organic fiber. Nevertheless, the multilayer wiring board finally produced by incorporating the reinforcing member of unwoven fabric such as glass cloth or organic fiber is found to increase in thickness undesirably, on the one hand, and cannot be easily formed with a fine via recess using a laser, on the other.
Accordingly, the object of the present invention is to provide a multilayer wiring board and a method of fabricating the same, which can improve the reliability of connection with the package members such as a semiconductor element mounted on the wiring board without using the reinforcing member composed of unwoven fabric such as glass cloth or organic fiber.
As the result of studying a solution to the aforementioned problem, the present inventor has developed this invention based on the discovery of the fact that a resin sheet, with a conductor pattern formed on one surface thereof, comprising a pair of film-like thermosetting resin layers and a resin film having a lower coefficient of linear expansion than, and sandwiched between, the film-like thermosetting resin layers is thermally expanded/contracted to a lesser degree during the process of fabricating a multilayer wiring board, thereby making it possible to improve the reliability of connection between the resulting multilayer wiring board and the package members such as a semiconductor element.
According to one aspect of the invention, there is provided a multilayer wiring board comprising a plurality of conductor patterns stacked with an insulating layer composed of a thermosetting resin interposed between adjacent conductor patterns, wherein the insulating layers each include a pair of film-like thermosetting resin layers and a resin film having a lower coefficient of linear expansion than, and sandwiched between, the thermosetting resin layers, and wherein the electrical connection between the stacked conductor patterns is established by vias formed through each insulating layer.
According to another aspect of the invention, there is provided a method of fabricating a multilayer wiring board comprising a plurality of integrally-stacked resin sheets each having one surface thereof formed with a conductor pattern;
wherein each of the resin sheets includes a pair of film-like thermosetting resin layers adapted to exhibit the bonding properties when heat treated at a predetermined temperature, a resin film having a lower coefficient of linear expansion than and sandwiched between the thermosetting resin layers, a conductor pattern formed on one surface of the sandwich, and vias having an end thereof coupled to the reverse surface of the conductor pattern and the other end thereof exposed to a second surface of the sandwich,
the method comprising the steps of:
stacking a plurality of the resin sheets in such a manner that the exposed end of the via of each resin sheet is in contact with a predetermined portion of the conductor pattern of an adjacent resin sheet; and
heat treating the resin sheets at a temperature enabling the thermosetting resin layers to exhibit the bonding properties thereby to integrate a plurality of the resin sheets.
According to this invention, a metal is filled by plating in each of the recesses, which are formed in the resin sheet and have the bottom thereof exposed to the reverse surface of the conductor pattern of the resin sheet, thereby forming vias with an end thereof coupled to the reverse surface of the conductor pattern and the other end thereof fixed at a predetermined portion of the conductor pattern of an adjacent resin sheet by a metal solder material or the like. In this way, the vias and the conductor patterns can be connected securely to each other.
The via and the conductor pattern can be fixed to each other easily by forming a metal solder layer at the end of the via.
The metal solder material desirably has a melting point higher than the thermosetting temperature of the thermosetting resin. The use of this metal solder material makes it possible to heat treat a plurality of resin sheets in stack in such a manner that the heat treatment temperature is increased to higher than the melting point of the metal solder material and then decreased to and held at the thermosetting temperature of the thermosetting resin for a predetermined length of time, or alternatively, the heat treatment temperature is increased to and held at the thermosetting temperature of the thermosetting resin for a predetermined length of time, and after the thermosetting resin layer is set, the temperature is increased to higher than the melting point of the metal solder material, followed by being decreased. In this way, the via and the conductor pattern can be connected positively to each other without causing any outflow of, or a void in, the metal solder material.
A resin film having a coefficient of linear expansion of not more than 20xc3x9710xe2x88x926/K at room temperature can be suitably used.
According to this invention, a multilayer wiring board comprising a plurality of resin sheets in a stack, each having a conductor pattern on one surface thereof, is fabricated. Each resin sheet includes a pair of film-like thermosetting resin layers adapted to exhibit the bonding properties when heat treated at a predetermined temperature, and a resin film having a lower coefficient of linear expansion than and sandwiched between the thermosetting resin layers. Even in the case where the resin sheets are heated during the fabrication process of the multilayer wiring board, therefore, the thermal expansion/contraction of the resin sheet can be reduced as compared with a resin sheet composed of a thermosetting resin layer alone.
With the multilayer wiring board according to the invention, therefore, the difference in thermal expansion/contraction between the wiring board and the package members such as the semiconductor elements can be reduced, thereby improving the reliability of connection between the semiconductor elements and the package members.